1. Field of the Invention
The present disclosure relates to a protection relay and an operation method of the protection relay, and more particularly, to a protection relay and an operation method of the protection relay for enhancing the relay operation of a frequency change rate relay element.
2. Description of the Conventional Art
In general, various protective relays may be used to protect feeder lines, motors, transformers, and the like in a power system. Such a protective relay can be classified into a mechanical type and a digital type. The digital protection relay may include the function of a relay, communication, a digital input/output contact point, and the like in a complex manner. When a failure occurs at any position of the power system, such a digital protection relay performs a role of quickly separating a section where a failure occurs. In particular, if a side of the power system is separated from the system while the distributed power generation is operated in interconnection with the power system, a frequency change rate relay element (81R) of the digital protection relay should be designed to be separated from the distributed power generation side or system. If not so, then there is worry of making asynchronous power due to reclosing, and moreover, it may cause a safety accident. In other words, if the value is equal to or greater than a prestored set value while monitoring a frequency change rate of the frequency, then the frequency change rate relay element may separate the distributed power generation therefrom in a quick and accurate manner to rapidly cope with a failure or disturbance of the power system.
A zero-crossing scheme may be typically used for a method of measuring the frequency of an input signal. The zero-crossing scheme measures a frequency by detecting a time point at which the zero point is crossed from a waveform of the input voltage or current signal and calculating an interval between the detected zero-crossing time points. However, frequency measurement using such a zero-crossing scheme may have a structure that is fragile to a noise included in the input voltage signal. In particular, if an external noise is induced to allow the input signal to be deviated from the zero-crossing point, then a large error occurs between the measured frequency and the actual frequency, thereby causing a malfunction of the frequency change rate relay element (81R).